Diffusive thermal waves have attracted considerable attention for use in non-invasive and non-destructive imaging tools in numerous fields of science and technology. As an example, these fields can include microelectronics, material process control, biomedical imaging, environmental remote sensing, and navigation. Experimental studies have been conducted for the reflection and refraction of thermal waves at the interfaces separating materials with different thermal properties. As a result, mathematical formulations have been derived for reflection and transmission coefficients of thermal waves at planar interfaces, and the total reflection and the associated critical angles of those waves have been explored. Accordingly, thermal properties of materials can be obtained and can be implemented for a variety of applications, such as thermal target classification, non-destructive testing, and biomedical imaging.